Subject-matter of the invention is a process for producing organically modified gels selected from lyogels or aerogels, by (i) emulsifying a basic, polar phase comprising water and starting materials for silicatic gels in an apolar phase comprising a water-immiscible precursor of an active silylating agent, (ii) commencing gel formation and aging by lowering the pH, and then (iii) lowering the pH to commence silylation and solvent exchange.
Against a background of increasing awareness of sustainable development and of the increasing expense of energy, and also of increasing scarcity of fossil raw materials, a higher position has been acquired by heat insulation for the purpose of energy saving. These requirements with regard to optimizing heat insulation protection are equally valid both for buildings, in other words for new-builds or existing buildings, and for thermal insulation in the logistical or fixed-location sector.
Against the backdrop of a sustainable insulation which features low thermal conduction and low combustibility, the focus is increasingly on inorganic, porous materials.
Aerogels with high porosities (>60%) and low density (<0.6 g/ml) have low thermal conductivity and therefore find a broad spectrum of application as thermal insulators (M. A. Aegerter et al. (Eds.), Aerogels Handbook Series: Advances in Sol-Gel Derived Materials and Technologies, 1st ed. 2011, Springer Verlag, New York Dordrecht Heidelberg London).
Gels, particularly SiO2 gels, are constructed of networks which are composed of primary particles which, after their linkage and sintering of the contact faces, in a sol-gel process, form stable, liquid-filled networks, known as lyogels. Whereas in the case of a lyogel the pores are filled with solvent, a hydrogel represents a special case of the lyogel, in which at least 50% of the pore liquid consists of water. These lyogels can be converted, by removal of the solvent, into aerogels. The pores of the aerogel, accordingly, are filled with air.
It is desirable to maximize the hydrophobicity of the SiO2 aerogels in order to reduce the water absorption and therefore the loss of the thermal insulating effect. Permanent hydrophobicity is achieved by treating the surface of gel networks with hydrophobic groups, preferably by modification.
This hydrophobization not only fulfills the purpose of reduced water absorption in the dry state, but also allows subcritical drying of the gels. In subcritical drying, the capillary forces which act result in contraction of the gels, this contraction being irreversible in the case of hydrophilic surfaces with free Si—OH groups through the condensation of these Si—OH groups in the contracted state. Hydrophilic lyogels, moreover, are generally filled with polar solvents, which are responsible, by means of high capillary forces, for severe contraction of the gels. In the case of the hydrophobized surfaces, the lyogels are generally present in apolar solvents. As a result of the drying, reduced contraction is observed, on the one hand, and the possibility for condensation of the Si—OH groups is prevented as well, these groups having been functionalized by the hydrophobization.
For economically viable implementation of a process for producing organically modified aerogels which can be dried subcritically it is particularly desirable to develop extremely rapid and cost-effective operating steps which are distinguished by efficient use of materials and, in particular, by short times and ease of implementation for the individual process steps.
In EP 0 948 395 B1, accordingly, a process for producing organically modified aerogels was developed by directly surface-modifying a hydrogel without first replacing the aqueous pore liquid by organic solvents. The examples use a sodium silicate solution or silicon tetrachloride as SiO2 source and employ hexamethyldisiloxane (HMDSO), trimethylchlorosilane (TMCS) or trimethylsilanol for modification. The free, hydrophilic Si—OH groups of the hydrogel react with the silylating agents used, and functionalization takes place with oxygen-bonded trimethylsilyl groups (TMS, (CH3)3SiO1/2). If the silylation is conducted with reaction of some of the water in the pores of the hydrogel with the silylating agent used (e.g., TMCS) to give the water-insoluble hexamethyldisiloxane, then the volume of the compound formed necessarily displaces at least some of the water from the pores. This leads, during the silylation of the internal surface of the network, to a simultaneous, complete or partial exchange of the liquid in the pores of the hydrogel for the water-insoluble, apolar medium.
A disadvantage of the process disclosed in EP 0 948 395 B1 is that either the hydrogels must be prepared from alkali metal silicate by prior desalting with ion exchangers, or the removal of the salts after silylation is difficult to accomplish. Upstream production of sol by neutralization of the alkali metal silicate solution constitutes an additional operating step which, because of the necessary cooling, for example, is energy-intensive and is costly and inconvenient as a process. Moreover, the process has the disadvantage that either the silylation takes place at temperatures of 80-100° C. or a very long reaction time of several days is required. Only through the use of large quantities of HCl and/or trimethylchlorosilane is success achieved here in a rapid and complete silylation. During the hydrophobization, the pore liquid is displaced from the gel and replaced by HMDSO; the authors of this patent, F. Schwertfeger and D. Frank, in a subsequent publication with M. Schmidt in Journal of non-Crystalline Solids (vol. 225, pp. 24-29, 1998), specify that for complete replacement of the pore liquid, they require at least 15 mol % of TMCS based on the pore water, corresponding to 81.5 g of TMCS per 100 g of hydrogel (see sample 2 in tab. 1), in order to obtain complete replacement of the pore liquid and hence to obtain aerogels of low density (less than 140 kg/m3). This results in 80 ml of HMDSO byproduct per 100 g of hydrogel. Raw materials costs here are high.
US 2012/0225003 describes a process for producing aerogel powders by mixing alkali metal silicate with an inorganic acid, preferably nitric acid, and an organosilane, preferably hexamethyldisilazane (HMN), and carrying out emulsification in an organic solvent, the selection here being specified from n-hexane, n-heptane, toluene, xylene or mixtures thereof. Both gel formation and solvent exchange take place simultaneously. A feature of the process is that it is very quick to implement, since the formation of sol, in other words the neutralization of the alkali metal silicate solution, takes place directly in the continuous phase. Gel formation and water displacement can be concluded here in about an hour. The removal of the electrolytes—that is, of the salts formed from the precursors, the silylating agent and the mineral acid—is accomplished in the course of the water displacement.
A disadvantage of this process is that the water-miscible silylating agent used, hexamethyldisilazane, is added in large quantities (60 ml per 500 ml of alkali metal silicate solution) and is consumed fully by the reactions which take place. Even the fraction which does not react with the surface in the course of the silylation is consumed by hydrolysis and condensation processes and cannot be directly recovered. The associated high costs for the raw materials, and the complexity of physical separation and/or disposal, result in high production costs.
The object was therefore to provide a rapid and economically viable process which is notable for efficient use of materials, particularly of the silylating agent, and which produces organically modified aerogels rapidly and easily.